                1. Field of the Invention        
The present invention refers to a device for monitoring the position of the rotor of an electric motor, in particular of a brushless motor, and the corresponding method.
2. Description of the Related Art
A DC brushless motor consists of a permanent magnet and of a stator consisting of a certain number of windings (generally three) normally star or polygon connected (for example triangle or delta). The windings are driven by means of a driving circuit the output stage of which generally comprises a half bridge (two bipolar or MOS transistors) for each winding.
FIG. 1 shows a typical circuit diagram of an output stage 1 and a motor 2 connected to it with three star-configured phases; the motor is indicated by means of three star-connected windings La, Lb and Lc. Detection of the position of the rotor during rotation generally involves the output stage of a winding of the motor, for example the output stage of winding La, being put into high impedance status, and the use of a circuit 3 suitable for detecting a Pbemf signal showing the polarity of the induced back-electromotive force (BEMF) of said winding in order to be able to detect zero crossing or ZC of the BEMF. This indicates the instantaneous position of the rotor whereas speed is calculated simply as the time distance of the last two ZC measurements. These data are thus used for driving the motor in a synchronous manner. The type of detection of position and speed is very applicable to motors the mechanical construction of which is near the ideal, i.e., motors whose poles are as equal as possible throughout the entire mechanical turn.
A first solution to the problem of optimizing driving also in the case of pairs of dissimilar polar couples is known from U.S. patent application Ser. No. 2004/0154411AI. In said patent application a method is disclosed for detecting the angular position of the rotor of a brushless motor in which detection circuitry associated with the motor supplies a signal showing the polarity of the BEMF. The method teaches the use of a two-directional counter for counting the difference in the time periods in which logic states 1 and 0 are present at the output of the detecting circuit. A method was used to eliminate the noise that could occur on the ZC signal and simultaneously a simplified form of filter was used, which was also useful for reducing the impact of the non-perfect mechanical alignment of the polar couples. This implementation led to noise filtering, but also constituted a delay in detection of the ZCs, which delay impacted the algorithms that were designed to monitor motor speed.